A refrigeration cycle system has been proposed which includes an internal heat exchanger that allows a high-pressure refrigerant flowing from the outlet of a condenser to expansion means to exchange heat with a low-pressure refrigerant flowing from the outlet of an evaporator to the suction inlet of a compressor. Heat exchange between the high- and low-pressure refrigerants in the internal heat exchanger allows evaporation of a liquid refrigerant flowing from the outlet of the evaporator, thus preventing both return of an excessive amount of liquid refrigerant to the compressor (to be referred to as “liquid back” hereinafter) and burn of the compressor due to a reduction in concentration of lubricating oil. In addition, increasing the difference between the enthalpy at the outlet of the evaporator and that at the inlet of the evaporator reduces the amount of refrigerant circulated, thus improving COP (the quotient of the cooling capacity or heating capacity divided by an input value) (refer to, for example, Patent Literature 1).